1. Field of the Invention
This invention relates to the use of specific compounds for curing blocked polyisocyanates in a coating composition.
2. Related Art in the Field
Blocked polyisocyanates are used to make one component coating systems which should be storage stable at room temperature. A mixture of a polyisocyanate and an active hydrogen containing compound is only stable in storage at room temperature and is only workable to form a coating if the reactive isocyanate groups are blocked and are, therefore, unable to react. When heat is applied, the blocking agents must of course split off and the isocyanate react with the active hydrogen containing material to form the cured polyurethane coating.
The blocking of polyisocyanates is a procedure which is well known for the temporary retardation of the reaction between the isocyanate groups and an active hydrogen containing compound. The literature notes various blocking agents; for example, tertiary alcohols, phenols, acetoacetic ester, ethyl malonate, and others. Various methods of blocking and unblocking isocyanates are revealed in the prior art, including the following patents and publications.
U.S. Pat. Nos. 4,495,229; 4,624,996; 4,590,009; 4,491,663; and U.K. Patent Application GB No. 2 151 643A. A paper concerning the subject is entitled "Blocked Isocyanates in Coatings," by T. A. Potter, et al (1985). Many of these publications and patents describe the use of amines as well as hydroxyl containing materials to block isocyanates and also describe these same materials as curing agents.
If a highly reactive material such as primary amine is used as the curing agent to unblock the isocyanate, the stability of the mixture of blocked isocyanate and curing agent is decreased because of the high reactivity of the amine toward the isocyanate even though this isocyanate is blocked. Therefore, premature gelling indicating reaction is a problem and storage becomes difficult and very sensitive to temperature. However, if a polyol is used as the curing agent, then the material will require an exceptionally high temperature for curing and perhaps a lengthy curing time as well. It is believed that the invention herein solves these problems.